Interchannel noise suppressor circuits



March 3, 1953 Filed NOV. 24, 1948 B. S. VILKOMERSON INVENTOR BENJHIIIN EVILKumEnEuN ATTORNEY March 3, 1953 B. s. vlLKoMERsON INTERCHANNEL NOISESUFPRESSOR CIRCUITS 2 SHEETS-SHEET 2 Filed NOV. 24, 1948 lNvEN'roRBENIHmN vinlmrmnslm ATTORNEY Patented Mar. 3, 1953 UNITED INTERCHANNELVNOISE SUPPRESSOR CIRCUITS Benjamin S. Vilkomerson, Camden, N. J.,assignor to Radio Corporation of America, a corporation of DelawareApplication November 24, 1948, Serial No. 61,864

3 Claims.

This invention relates to muting circuits for broadcast receivers andparticularly to combined interchannel noise suppressor and tuningindicator circuits for amplitude-modulated (am) or frequency-modulated(f-rn) carrier wave re ceivers.

It has long been recognized that it is desirable to mute the audiochannel of a broadcast re ceiver while the receiver is tuned betweenadjacent channels. Furthermore, the receiver should be muted when theamplitude of the received carrier wave falls below a predetermined valuewhere the wave would be below marginal utility. Another requirement of anoise muting or interchannel noise suppression circuit is that the audiochannel should not be opened by intermittent noise pulses even when theyhave high peak amn plitudes. On the other hand, when the audio channelis opened by a carrier wave of sufcient amplitude it should not closeagain when theV amplitude of the carrier wave decreases momen-j tarily.A further requirement of an interchannel noise suppressor circuit isthat the audio channel should be suddenly opened completely and shouldnot remain in' a semi-conducting condition when a carrier Wave ofmarginal utility is received. Numerous muting circuits have been devisedheretofore but none of them meets all the above requirements.

The importance of the proper tuning of an f-m receiver has also longbeen recognized. In order to secure the best signal-to-noise ratio andthe minimum amount of audio distortion the receiver should be tuned sothat the center frequency of the f-m wave coincides with the assignedcenter frequency of the discriminator which, in turn, should be equal tothe frequency of the center of the passband of theintermediate-frequency coupling circuits. At the present time thefrequency departure of an f-m broadcast wave from its as-V signed centerfrequency amounts to a maximum of 75 kilocycles (kc.) in eitherdirection from the center frequency. Although the intermediate-frequencypassband usually has a width of 200 kc., which is more than the Width ofthe f-m wave band, the f-m wave should be accurately centered on theintermediate-frequency passband to allow for unavoidable drift of thelocal osci1- lator and to minimize the effect of non-linearity in thediscriminator characteristic. It is just as important to provide propertuning of an a-m receiver in order to prevent distortion of thereproduced signal.

It is accordingly an object of the present in vention to provide novelmuting circuits for a broadcast receiver, whereby the audio channel ofthe receiver is normally inoperative and is suddenly opened or renderedfully operative in response to the reception of a carrier wave having apredetermined and adjustable minimum average amplitude, the circuitbeing arranged in such a manner that even high intermittent noise peakswill not open the audio channel While a momentary reduction of theamplitude of the received carrier wave will not close a previouslyopened audio channel.

Another object of the invention is to provide a muting circuit of thetype described where the muting amplifier serves the additional functionof rendering a normally inoperative electronic tuning indicatoroperative in response to the re-I ception of a carrier wave. N

A further object of the invention is to provide a combined interchannelnoise suppressor and tuning indicator circuit for an a-m or f-m re-`receiver where the muting amplifier of the noise suprressor circuit willcontrol the tuning indicator tube to render it operative when a carrierwave is received and where further means are provided to indicatevisually the extent and sense of mistuning of the receiver.

A modulated carrier wave receiver in accordance with the presentinvention may include a detector and a normally inoperative audioamplifier, the detector having a common cathode with the audioamplifier. The load circuit of the detector controls a mutingamplifierwhich is normally conducting. Furthermore, a predetermined bias voltageis impressed on the control grid of the muting amplifier. The cathodecircuits of the muting amplier and of the combined detector and audioamplifier have a common impedance. Consequently, when a carrier wavewhose positive peak value exceeds the cathode potential is received, thenegative direct current voltage developed across the detector loadcircuit biases the grid and thus reduces the space current flowingthrough the muting amplifier. This will, in turn, reduce the cathodepotential of the detector so that the relative peak radio-frequencypotential of the signal will increase. Accordingly, a largerdirect-current voltage is developed across the detector load circuitwhich further reduces the space current through the muting amplifier.The muting amplifier is thus suddenly cut off while the audio amplierchannel is just as suddenly opened. The predetermined bias voltageapplied to the muting amplifier control grid determines the minimumaverage peak amplitude of a carrier wave which will open the audiochannel.

The muting ampliiier may be coupled to an electronic tuning indicatortube to render its target bright when a carrier Wave is received. Thetuning indicator tube will indicate the amount and sense of mistuning ofthe receiver. Furthermore, the muting circuit and indicator tube of theinvention may be applied to an FM receiver.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation, asWell as additional objects and advantages thereof, will best beunderstood from the following description when read in connection withthe accompanying drawings, in Which:

Fig. 1 is a circuit diagram, partly in block form, of an a-m receiverembodying the present invention;

Fig. 2 is a circuit diagram, partly in block form, of an f-m receiverincluding a combined mutingV and tuning indicator circuit in accordancewith the present invention;

Fig. 3 is a graph illustrating voltages developed at different points ofthe receiver of Fig. 2;

Figs. 4 to 7 are plan views of the target of the electronic shadowindicator tube included in the circuit of Fig. 2 Which illustrate theappearance of the target when the receiver is mistuned (Figs. 4, 5 and6) or properly tuned (Fig. 7) and Fig. 8 is a circuit diagram of aportion of an a-m receiver embodying a modified muting and tuningindicator circuit in accordance with the invention.

Referring now to the drawings in which like components have beendesignated by the same reference numerals, and particularly to Fig, lthere is illustrated an a-m receiver comprising antenna I forintercepting an a-m wave. The wave may be amplified by one or moreradio-frequency (r-f) amplifiers 2 and converted to anintermediate-frequency (i-f) wave by frequency converter 3 including abeat frequency or local oscillator. R-f amplifier 2 and the localoscillator of frequency converter 3 are tunable to the desired frequencyby variable reactances such as capacitors 4 andv 5. Capacitors 4 and 5are variable in unison by tuning control knob E in accordance withconventional practice.

The i-f wave derived from converter 3 may be amplified by i-f amplier 1having a tuned output circuit 8 magnetically coupled to input circuit IUof the detector. The detector includes tube II which preferably is acombined diodetriode as illustrated. Tube II has a common cathode I2 forits diode and triode sections. Anode I3 forms the diode section withcathode I2 while the triode section further includes control grid I4 andanode I5.

A detector load circuit Iii consists of resistor I1 shunted by capacitorI8 and is arranged in series with input circuit I3 and effectivelybetween anode I3 and cathode I2. Actually, one terminal of load circuitI6 is grounded while cathode I2 is connected to ground through selfbiasnetwork including resistor 2| shunted by capacitor 22. An automatic gaincontrol voltage (AVC voltage) is derived across load circuit I6 and maybe impressed through resistor 24 and leads 25, 25 on r-f amplifier 2 andi-f amplifier 1 in accordance with conventional practice. A positivedelay voltage for the AVC circuit is introduced through droppingresistor 26 having one terminal connected to a suitable voltage sourcewhich may have a voltage of 100 Volts,

4 indicated at +B, while its other terminal is connected to leads 25.The purpose of the AVC delay voltage will be explained hereinafter.

The modulation or audio signal is also developed across detector loadcircuit I5. The audio signal Without its direct current component isimpressed upon control grid I4 of the triode amplifier section For thispurpose the high potential terminal of detector load circuit I5 isconnected to ground through audio coupling capacitor 28 andpotentiometer 30 which functions as the volume control. The audio signalis obtained through variable tap 3l connected to control grid I4 throughaudio coupling capacitor 32. Grid leak resistor 33 is connected betweencontrol grid I4 and ground.

In accordance with the present invention there is provided control ormuting amplifier 35 having cathode 35, control grid 3i' and anodeCathode 36 is connected to cathode I2 of diode-triode II. The directcurrent component of the signal voltage as developed across load circuitI5 is impressed on control grid 31 through resistors 40, 4I connected tothe high potential terminal of load circuit I6.

The control grid 31 of muting amplifier 35 is also supplied-with apredetermined bias voltage. To this end there is provided apotentiometer including resistors 4'2 and 43 connected in series betweena positive voltage source indicated at +B and a negative voltage sourceof -25 volts as indicated. By means of variable tap 44 on resistor 43 avoltage of predetermined polarity and magnitude may be derived andirnpressed through resistors 45 and 4I on control grid 3l. Actually, theseries combination of resistors 45, 4I] and I1 forms a voltage dividerhaving one terminal connected to ground, the direct current component ofthe signal developed acdoss load resistor I1 being in series with thevoltage of potentiometer 43.

Anode 33 of muting amplifier 35 is connected to +B through droppingresistor 41. A double vane indicator tube 48 of the electronic shadowtype has its target 50 connected to anode 38 of muting tube 35. The twocontrol rods 49, 5I of indicator tube 48 are connected together.Resistor 52 is provided between target 55 and control rods 49, 5I, whileresistor 53 connects the control rods to ground.

Anode I5 of the triode section of tube I I is connected to +B throughload resistor 54 across which the amplified audio signal is developedwhich is coupled through capacitor 55 to audio amplifier 56 andreproduced by loud speaker 51.

The muting circuit of the receiver of Fig. 1 operates as follows. Thebias applied to the control grid 31 of muting amplifier 35 is adjustedby means of tap 44 to such a .value that the amplifier 35 is normallyconducting. Muting amplier 35 preferably has a high mutual conductanceso that a large variation of its plate current is caused by a variationof its grid voltage. Furthermore, audio amplifier I2, I4, I5 has ahigher amplification factor than that of muting amplifier 35. A highamplification factor as distinguished from the mutual conductance meansthat the plate voltage Variation due to a variation of the grid voltageis large; hence a relatively small change in grid bias will change thetube from the amplifying to the cut-off condition.

The space current which normally iiows through muting amplifier 35 willdevelop a predetermined voltage across bias network 2U Which may be ofthe order of 5 volts. Accordingly. audio amplifier I2, I4, I5 iscompletely biased off because its control grid is 5 volts negative withrespect to its cathode and the tube characteristic is such that onlyabout 3 volts is necessary to cut off its plate current. Let it now beassumed that an amplitude-modulated carrier wave is tuned in by rotationof tuning control knob 6. If the average peak amplitude of the a-m waveimpressed through input circuit I on detector I2, I3 exceeds 5 volts,detection will take place. If the average peak amplitude of the receivedwave is less than volts, no rectification can take place because cathodeI2 is 5 volts above ground while anode I3 is normally, that is, in theabsence of a signal, substantially at ground potential.

As soon as the received carrier wave is detected, an audio signal isdeveloped across detector load circuit I6. The rectified signal currentwill develop a voltage of negative polarity at the high potentialterminal of load circuit I6, and this negative voltage is impressedthrough resistors 4U and 4I upon control grid 31 of muting amplifier 35.Consequently, the space current flowing through muting ampliiier 35 isreduced thereby reducing the voltage drop across bias network 20. Theresult is that the positive voltage of cathode I2 with respect to groundis reduced. This will increase the eiiiciency of rectification of diodeI2, I3 thereby causing a larger negative voltage to be app-lied to thecontrol grid of muting tube 35 which is thus very rapidly cut off. This,in turn, will render the audio amplier I2, I4, I5 suddenly conductingbecause its cathode potential approaches ground potential which is thenormal potential of grid I4.

The audio channel is accordingly instantaneously opened as soon as awave is received having a mean amplitude which is larger than apredetermined minimum value. This minimum value is adjustable by meansof tap 44 so that carrier waves of less than marginal utility can beprevented from opening the audio channel. The audio channel will opensuddenly so that it is either fully opened or fully closed and nosemiconducting condition can exist.

The time constant of resistor 4I and capacitor 3S in the grid circuit ofcontrol grid 31 is large compared to a cycle of the modulation signaland may be of the order of .1 second. Conseouently, intermittent noisepulses of high peak amplitude cannot open the audio channel because theycannot develop a negative voltage across resistor 4I and capacitor 39which is sucient to cut off the muting amplifier. On the other hand,even relatively weak continuous carrier waves will be able to open theaudio channel as long as their mean amplitude exceeds the predeterminedminimum value.

The delay voltage iniected through resistor 26 on AVC leads 25, has thefollowing function. Normally, that is, in the absence of a carrier waveor in the presence of a carrier wave below the predetermined minimumamplitude, no AVC voltage is developed. Accordingly, the resonantcircuits cn which the AVC voltage is impressed, are damped due to thesmall residual bias of the grids, and this will reduce the amplificationin r-f amplifier channel 2 and in i-f amplifier channel 1. However, assoon as a carrier wave is received which has an average amplitude abovethe predetermined minimum value, an AVC voltage is developed which willovercome the delay voltage. Now, the resonant circuits to which the AVCvoltage is applied are no longer damped so that the 6 amplification ofthe wave through the r-f and i-f channels is increased. This action ofthe AVC circuit will further aid in suddenly opening the audio channel.

Tuning indicator tube 48 is normally inoperative, that is, as long asmuting amplifier 35 is conducting space current. In that case, thevoltage drop across resistor 41 will lower the voltage of target 50 tosuch an extent that electrons are not attracted by the target and thetarget will remain dark. However, when muting amplifier 35 is cut off,the voltage applied to target 50 becomes sufiiciently high to attractthe electrons. Control rods 49, 5I are connected together and maintainedthrough potentiometer 52, 53 at such a positive potential that they willnot cast a shadow on target 50. Thus, every time a station is tuned in,target 50 will light up to indicate the presence of a carrier Wave ofsufficient amplitude even if volume control tap 3| is turned down sothat no sound is heard from loud speaker 51.

Fig. 2 illustrates an f-m receiver provided with the muting circuit ofthe present invention. The tuning indicator circuit included in thereceiver of Fig. 2 has been disclosed and claimed in the copendingapplication to B. S. Vilkomerson, Serial No. 24,371, filed on April 30,1948, now U. S. Patent 2,502,293 issued on March 28, 1950 and assignedto the assignee of this application. An f-m wave may be intercepted byantenna I and amplified by r-f amplier stage 2. The r-f wave derivedfrom amplifer 2 is converted by frequency converter 3 to an i-f wavewhich is further ampliiied by one or more i-f amplifiers 1. R-f amplier2 and the local oscillator of frequency converter 3 are tunable to thedesired frequency by variable reactances such as capacitors 4 and 5,variable in unison by tuning control knob 6 in accordance withconventional practice.

The last if amplier stage 'l has an output circuit 3 comprising coil 60tuned by capacitor 6I. The inductance of coil 60 may be adjusted bypara-magnetic core 62. If output circuit S forms the primary circuit ofa frequency discriminator network which includes secondary circuit 63comprising coil 64 tuned by capacitor 65. Primary circuit 4.- andsecondary circuit 63 are coupled to each other by link circuit 66comprising coil 61 and capacitor 63 arranged in series. Coils 6I] and 61are magnetically coupled as indicated. One terminal of coil 51 isconnected to coil 6d at a tap on one side of its midpoint. The otherterminal of coil 61 is coupled through capacitor 68 to another tap oncoil 64 which is closely adjacent but on the other side of the midpointof coil Primary circuit 3 and secondary circuit 63 preferably areindividually magnetically shielded. Thus, coils 66 and 6d are notmagnetically coupled but are coupled through link circuit 66. By meansof paramagnctic core 1I), coil 64 may be electrically balanced.Paramagnetic core 1i serves the purpose of adjusting the resonantfrequency of secondary circuit 63. The balance of coil i3d. may have tobe readiusted by core 'I6 after the circuit is tuned by core 1 I.

The frequency discriminator network including primary circuit 8 andsecondary circuit 63 forms part of a frequency discriminator.Preferably, the frequency discriminator is of the ratio detector typewhich has been described by Stuart Wm. Seeley on page 26 of the December1947 issue of FM and Television. The ratio detector illustrated in Fig.2 comprises two rectifiers such as vacuum diodes 12 and 13. The cathodeof diode 12 is connected to one terminal of secondary circuit 53 whileits other terminal is connected to the anode of diode 13. Stabilizingcapacitor 'it is connected between the anode of diode 12 and the cathodeof diode 13. Stabiliaing capacitor 14 is shunted by resistors 15, 15having their junction point grounded as shown. Stabilizing capacitor 14presents a low impedance to intermediate-frequency andmodulation-frequency currents. stabilizing capacitor 14 and resistorsV15, 16 are chosen in such a manner that they have a time constant whichis larger than a cycle oi the modulation frequency and which may be ofthe order of 0.1 second. Capacitor 11 bypasses resistor 15 and has a lowimpedance to intermediate-frequency currents but a high impedance tomodulation-frequency currents.

The ratio detector of Fig. 2 operates in a conventional manner. Thefrequency discriminator network is balanced at the center frequency,that is, at the center frequency the carrier frequency voltagesimpressed upon diodes 12 and 13 are of equal magnitude. Accordingly, thevoltage at the junction point o coil 61 and capacitor 63 will be equalto that of the junction point of resistors l5, 1t' which is at groundpotential. However, when the instantaneous frequency of the f-m wavedeviates from its center frequency, the frequency discriminator networkbecomes unbalanced the voltage impressed upon diode 12 Jill be unequalto that impressed upon diode 13. Accordingly, the voltage of thejunction point of capacitor 58 and coil 61 will no longer be Zero withrespect to ground.

When an f-m wave is received, a voltage is developed across stabilizingcapacitor 14 and resisters l5. "iii which varies slowly with changes inamplitude of the received wave. Thus, an AVC voltage may be derived fromlead 1t which is connected to the junction point between resistor 15 andthe anode of diode 12. The AVC voltage may be impressed in aconventional manner through filter resistor 85 upon i-f amplifier 1 andr-f amplier 2, as indicated, by leads 3|, 8|.

The demodulated or audio signal may be obtained from lead 82 connectedto the junction A point between coil 61 and capacitor 68 and fed throughrie-emphasis network 83 and coupling capacitor 84 to audio amplifier 35.De-emphasis network 83 comprises series resistor 8S and shunt capacitorS1. It is conventional practice to provide apre-emphasis network in oneof the audio amplifier of an f-m transmitter which attenuates audiofrequencies below 1500 cycles more than higher frequencies. De-ernphasisnetwork 83 is conventionally provided in an f-m receiver for the purposeof attenuating audio frequencies above 1500 cycles more than the loweraudio frequencies to establish again the proper balance of the audiosignal.

The audio signal derived from de-emphasis network 83 and couplingcapacitor 84 is developed across potentiometer 33 connected betweencoupling capacitor Bil and ground. The audio signal may be taken frommovable Volume control tap 90 and coupled through coupling capacitor 3|to control grid 92 of audio amplifier 85. Audio amplier S also has acathode 94 and an anode 95 connected to the anode voltage supply +Bthrough anode resistor 95. The amplified audio signal developed acrossanode resistor 95 is impressed through coupling capacitor 91 on audioanipliiier H80 and may be reproduced by loud speaker |0|.

The output signal of the ratio detector is utilized to obtain anindication of the amount and senseof mistuning ofthe receiver. To thisend the output signal of the ratio detector is iiltered by low-pass oraudio lter network |03 comprising series resistor |04 connected to thejunction point between de-emphasis network 83 and audio couplingcapacitor 84, and shunt capacitor |05. The signal derived from filternetwork |03 is the mean voltage or direct-current voltage component ofthe rectified signal, that is, of the audio signal. This mean voltagewill be zero only when the receiver is properly tuned, and will beeither positive or negative when the receiver is tuned to a frequencyabove the assigned center frequency of secondary circuit 63 or belowthis assigned frequency, respectively.

This means voltage is now impressed on direct current ampliiier |06including cathode |01, control grid |08 and anode H0. Control grid |03is connected to filter network |03. Anode ||0 is connected to a suitablepositive voltage supply indicated by +B through resistor lead ||2 andanode resistor 41. Cathode |01 is also connected to +B through resistor||3, lead ||2 and resistor 41, and is connected to ground throughcathode resistor ||4 which may be adjustable as shown. Resistors 3 and||4 accordingly function as a voltage divider which will keep cathode|01 at a predetermined and adjustable positive voltage.

The control signal developed across anode resistor l is impressed on oneof the control rods or varies 49 of a double vane indicator tube 48 ofthe electric shadow type which may be a 6AF6 tube. Indicator 48 has agrounded cathode, two control rods 49, 5| and a fluorescent target 50connected to +B through resistor 41, as shown. Control rod 49 isconnected by lead 5 to the anode H0 of amplifier |06. The junction pointbetween anode resistor 41 and target 50 is connected to control rod 5|through resistor 52, and control rod 5| is connected to ground throughresistor 53. Resistors 52 and 53 accordingly form a voltage dividerwhich will maintain control rod 5| at a predetermined positive voltage.

Control rod 5| and target 50 of indicator tube 48 are connected to theoutput of muting amplier |20. The cathode |2| of muting amplifier |20 isgrounded through resistor |22 and bypassed to ground by capacitor |23,while its control grid |24 is connected through resistor |25 to lead 18which carries the AVC voltage. The cathode |2| of muting amplifier 20 istied to the cathode 94 of audio amplifier 85, while the anode |26 ofmuting amplifier |20 is tied to target 50 of indicator tube 48.

The f-m receiver of Fig. 2 including the tuning indicator and mutingcircuit of the invention operates as follows. In the absence of acarrier wave no AVC signal is developed and accordingly muting amplifier|20 will conduct current. This is due to the fact that its control grid|24 is connected to the anode voltage supply +B through resistor |28which, in turn, is connected to ground through resistors |25 and 15.Control grid |24A will accordingly have a potential which issuiiiciently positive to render muting amplifier |20 conducting. Theresulting voltage drop across anode resistor 41 will reduce the voltageof target 50 of indicator tube 48 to such an extent that the target willremain dark. At the same time, the space current of muting amplifier |20will ow through its cathode resistor |22. This, in turn, will raise thepotential of cathode |2| as well as that of cathode 94 of audioamplifier which are tied together. Consequently, audio amplifier 85 willbe biased to or beyond cut off. Target 50 of the tuning indicator 48willremain dark as illustrated in Fig. 4.

When a carrier wave is received, an AVC signal is developed by the ratiodetector which is impressed through resistor |25 on control grid |24 ofmuting amplifier |20. This voltage has been illustrated in Fig. 3 bycurve |33 and varies as a function of the mistuning of the receiver.Accordingly, a negative AVC voltage is impressed on control grid|24fwhich will cut off muting amplier |20. Hence, the space current ofmuting tube |20 will no longer flow through cathode resistor |22 so thatthe potential of both cathodes |2| and 34 will rise. Audio amplifier 85is now in a condition to conduct space current and to amplify the audiosignal impressed thereon. At the same time, the cutting off of mutingampliner |20 will raise the voltage of target 50 so that it can nowattract electrons. f

Let it now be assumed that tuning control knob 6 is so adjusted that thereceiver is mistuned and the center frequency of the received f-m waveis lower than the assigned center frequency value F1 of secondarycircuit 63. In that case, the demodulated signal which is impressed oncontrol grid |08 of amplifier |06 has a positive mean voltage asillustrated by curve |30 of Fig. 3. Curve |30 thus represents thevoltage impressed on control grid |08 as a function of the tuning of thereceiver. Since a positive voltage is impressed on amplifier |06 theamplifier will conduct more space current than normally with acorrespondingly larger voltage drop across its anode resistor Thevoltage impressed through lead ||5 on control rod 49 of indicator 48will therefore decrease. When the voltage of control rod 43 decreases,it will repel electrons and cast a shadow on target 50. The shadow |32appearing on target 50 is illustrated in Fig. 5 and its angle iscontrolled by rod 49 to indicate the amount of mistuning of thereceiver.

Let it now be assumed that tuning control knob 6 is so adjusted that thereceiver is mistuned and the center frequency of the f-m wave is higherthan the assigned center frequency value of secondary circuit 63. Nowthe mean voltage |30 of the audio' signal is negative, and this nega.tive voltage will be impressed upon control grid |08 of amplifier |06.The space current through amplifier |06 is thus reduced so that thevoltage impressed through lead ||5 on control rod 43 will rise. Controlrod 49 will therefore attract electrons and an overlapping brightportion |34' which is brighter than the remainder of the target willappear on target 50 as shown in Fig. 6.

If tuning control knob 6 is adjusted so that the receiver is correctlytuned, the mean voltage |30 of the audio signal (at the frequency F1) iszero as illustrated in Fig. 3. Accordingly, a vo1t age of Zero magnitudeis impressed uponcontrol grid |08 of amplifier |06. Cathode resistor ||4is adjusted in such a manner that cathode |01 has a predeterminedpositive voltage. Under these conditions the space current of amplifiertube |06 is of such a magnitude that the voltage of control rod 46 issuicient to cause either shadow .32 or overlapping bright portion |34just to disappear. Target 50 accordingly will be uniformly illuminatedas shown in Fig. 7. Muting amplifier |20 preferably has a lowamplification factor so that the brightness of target 50 depends to acertain extent on the strength of the received carrier wave. In otherwords, unless the AVC voltage |33 illustrated in Fig. 8 exceeds acertain 10 value, muting amplifier |20 will still conduct space currentto a certain extent. The brightness of target 50 thus indicates thestrength of a wave of marginal strength which is received.

It will be understood that the ratio detector which has been illustratedby way of example in Fig. 2 may be replaced by anyl conventionalfrequency discriminator. Furthermore, the AVC voltage which is impressedon muting amplifier |20 may also be derived in another conventionalmanner. Thus, it may be desired to derive the AVC voltage throughasharply tuned circuit so that audio amplifier 05 is only'rendered con`ducting when the mistuning of the receiver is less than a predeterminedamount.

It is .also to 4be understood that a muting and tuning indicator circuitof the type illustrated in Fig. 2 may be incorporated in an a-mreceiver. Such a circuit has beenillustrated in Fig. 8 to whichreference is now made. The a-m receiver of Fig. 8 comprises an i-foutput circuit 8 which may be connected to i-f amplifier 1 of Fig. 1.I-f circuit 8 comprises coil |35 inductively coupled to input circuit l0of the detector. The detector comprises diode I2, |3 having a loadcircuit I6 across which the audio signal is developed. The audio signalis then impressed upon control grid I4 of the audio amplier in themanner explained in connection with Fig. 1.

Muting amplifier 35 has its cathode 36 tied to cathode i2 of the audioamplier. Both cathodes are connected to ground through self-bias network20. Control grid 31 of muting amplifier 35 is biased at a predeterminedvoltage derived from voltage divider 42, 43. Anode 38 of vmutingamplifier 35 and target 50 of indicator 48 are also connected together.It will accordingly be obvious that the audio amplifier is controlled bymuting amplifier 35 in the manner explained in connection with Fig. 1,While the target 50 of in-f` dicator 48 becomes bright as soon as acarrier wave is received and demodulated. Furthermore, the audio channelof the circuit of Fig. 8 will not open in response to intermittent noisepulses as previously explained.

However, the circuit of Fig. 8 diners from that of Fig. 1 in thatcontrol rod 49 is not tied to conj trol rod 5| of indicator tube 48 butis made re`` sponsive to the amountand sense of mistuning of thereceiver. To this end i-f circuit 8 is pro--` vided With a second coil|36 which is coupledzto frequency discriminator network |31 of a Seeleylfrequency discriminator which has been dis` closed and claimed in U. S.Patent 2,121,103. Thev frequency discriminator network |31 includes'coil |38 tuned by capacitor |40. The midpoint of coil |38 is connectedthrough capacitor 14| to the high alternating potential terminal of i-fcircuit 8. i

Frequency discriminator network |31 functions in a well known manner. Aprimaryl or reference voltage is injected into the secondary circuit|38; |40 through capacitor |4|. At the same time asecondary voltage isdeveloped at each terminal of the circuit |38, |40. Since the phaseof-the two secondary voltages with respect to the primary voltage varieswith the difference in fre-3 quency of the impressed i-f wave and theresonant frequency of circuit ,|38, |40, a resultant: wave is developedat each terminal of the cir cuit which has an amplitude representativeofthis phase difference, which, in turn, is responsive to the frequencydifference. The two waves are now rectified by `diodes |42, |43 havingtheir anodes connected to respective terminals of secondary circuits|38, |40. The cathodes of diodes |42, |43 are interconnected throughload resistors 44, |45 Which are bypassed by capacitor |46 having a lowimpedance to intermediate-frequency currents. The cathode of diode |43is grounded while the junction point of load resistor |44, |45 isconnected to the midpoint of coil |38.

The voltage developed across load resistors |44, |45 is integrated orfiltered by filter network |03 in the manner explained in connectionwith Fig. 2. Accordingly, the voltage impressed on control grid |08 ofdirect current amplifier |06 is represented by curve |30 of Fig. 3.Anode H0 of amplifier |06 is connected by lead ||5 to control rod 49 asexplained in connection with Fig. 2. Control rod 49 will accordinglycast a shadow (Fig. 5) or a bright overlapping portion (Fig. 6) ontarget `50 when the receiver is mistuned above or below the correctfrequency. When the receiver is properly tuned target 50 will appear asshown in Fig. 7. The interchannel noise suppressor and tuning indicatorcircuit of Fig. 8 will accordingly give the same tuning indication asthat of Fig. 2. The circuits of Figs. 2 and 8 can accordingly' becombined to obtain an a-m `and f-m receiver Where identical tuningindications can be observed on the target 50 of indicator tube 40whether an a-m or an f-m wave is received. It will be obvious that sincecontrol rod 5| of indicator 48 is not required in the circuits of Figs.1, 2 and v8 it may beomitted.

It will be understood that the circuit specifications of theinterchannel noise suppressor and tuning indicator -circuit of theinvention may vary according to the design for any particularapplication. The following circuit specifications are included, by Wayof example only, for the circuit .of Fig. l;

Diode-triade amplifier 6AV6 type Muting amplier35 6AG5 type (connectedas a triode) Tuning indicator tube 48----- `6AF6 type Resistor L241,000,000 ohms Resistor 26 15,000,000 ohms Resistor |1 270,000 ohmsResistor 2| 1,200 ohms Resistor 54 270,000 ohms Resistor 30 1,500,000ohms Resistor 33 15,000,000 ohms Resistor 42 270,000 ohms Resistor 432,000,000 ohms Resistor 45 10,000,000 ohms Resistor 40 1,000,000 ohmsResistor 4| 1,000,000 ohms Resistor 41 24,000 ohms Resistor 52 560,000ohms Resistor 53 4390,000 ohms Capacitor |8 100 micromicrofaradsCapacitor22 5 microfarads Capacitor 28 .01 microfarad Capacitor 32 .01microfarad Capacitor -39 .1 microfarad Capacitor 55 .01 microfaradv'-Ihe following 'are illustrative circuit specifications for thecircuit of Fig. 2:

Diodes 12, 13 6AL5 Muting amplifier |20 6AG5 (connected as a triode)Tuning indicator tube 48 6AF6 Audio amplifier 05 6AV6 Amplifier |00`6ft/i6 (connected as a trio de) 12 Resistor 86 15,000 ohms Resistor 881,500,000 ohms Resistor 93 15,000,000 ohms Resistor 96 270,000 ohmsResistor 15 10,000 ohms Resistor 16 10,000 ohms Resistor |25 220,000ohms Resistor |28 8,200,000 ohms Resistor |22 680 ohms Resistor 4124,000 ohms Resistor 52 560,000 ohms Resistor 53 390,000 ohms Resistor|04 1,000,000 ohms Variable Resistor ||4 zero to-5,000 ohms Resistor ||3220,000 ohms Resistor 1,000,000 ohms Capacitor 14 5 microfaradsCapacitor 11 100 micromicrofarads Capacitor 81 .005 microfarad Capacitor84 .01 microfarad Capacitor 9| 01 microfarad Capacitor |23 5 microfaradsCapacitor 91 .01 microfarad Capacitor |05 .l mi'orofarad The circuitconstants of the circuit of Fig. 8 may be the same as those ofcorresponding components of the circuits of Figs. l and 2 as givenabove.

There has thus been described an interchannel noise Vsuppressor ormuting circuit which may Vbe used with an a-m or f-m receiver. Themuting circuit for an a-m receiver will suddenly render the audiochannel conducting When a Wave of predetermined average amplitude isreceived. The `audio channel Will not open on intermittent noise 'pulsesof high amplitude. On `the other hand, the audio channel will not closeAwhen it has been opened by an a-m carrier Wave when the amplitude ofthe carrier Wave Amomentarily decreases. The muting amplifier of thenoise suppressor circuit may serve the additional func-` tion ofrendering a tuning indicator tube operative or inoperative. Furthermore,Ythe tuning indicator circuit may include a frequency discriminator andamplifier for obtaining a visual indication of the sense and magnitudeof the mis-tuning of the receiver.

What is claimed is:

1. An angle-modulated carrier wave receiver comprising a carrier wavetransmission channel, a frequency discriminator coupled to said channeland having an output circuit ,for developing the modulation signalincluding a component representative of the sense and amount offrequency departure of the center frequency of said Wave from apredetermined frequency, means for separating said component `from saidsignal, circuit means coupled to said discriminator Ifor deriving acontrol voltage representative of the presence or absence of a carrierwave Areceived by said receiver, a modulation signal amplier coupled tosaid output circuit for amplifying said modulation signal, a rst controlamplifier responsive to said component, a second control ampliiierresponsive to said control voltage and arranged to be conducting in theabsence vof Aa carrier Wave, said modulation signal amplifier having ahigher amplification factor than said second control amplifier, saidsignal amplifier and said second control amplifier having each ac:.thode, said cathodes being connected together, an impedance elementin the common cathode circuit of said signal amplifier and said secondcontrol amplifier, said signal amplifier beingarranged to benon-conducting in the absence of a carrier Wave, whereby the presence ofa carrier wave will cause said second control amplifier to be cut offand will render said signal amplifier conducting, an electronicindicator device arranged to be inoperative in the absence of a carrierWave and coupled to said first control amplifier, and a circuitconnection between said second control amplifier and said device torender said device operative when said second control amplifier is cutoff.

2. An angle-modulated carrier wave receiver comprising a carrier wavetransmission channel, a frequency discriminator coupled to said channeland having an output circuit for developing the modulation signal whosemean voltage 1s proportional in polarity and magnitude to the sense andamount of frequency departure of the center frequency of said wave fromthe assigned center frequency of said discriminator, a filter networkcoupled to said output circuit for deriving said mean voltage from saidsignal, circuit means coupled to said discriminator for deriving acontrol voltage representative of the presence or absence of a carrierwave received by said receiver, a modulation signal amplifier coupled tosaid output circuit for amplifying said modulation signal, a firstcontrol amplifier responsive to said mean voltage, a second controlamplifier having an amplification factor that is lower than that of saidsignal amplifier' and responsive to said control voltage and arranged tobe conducting in the absence of a carrier wave, said signal amplier andsaid second control amplifier having each a cathode, said catho-desbeing connected together, an impedance element in the common cathodecircuit of said signal amplifier and said second control amplifier, saidsignal amplifier being arranged to be non-conducting in the absence of acarrier wave, whereby the presence of a carrier wave will cause saidsecond control amplifier to be cut off and will render said signalamplifier conducting, an electronic indicator device coupled to saidfirst control amplifier for indicat ing the magnitude and polarity ofsaid mean voltage, said device being arranged to be inoperative in theabsence of a carrier wave, and a circuit connection between said secondcontrol amplifier and said device to render said device operative whensaid second control amplifier is cut off.

3. A frequency-modulated carrier wave receiver comprising a carrier wavetransmission channel, a frequency discriminator coupled to said channeland having an output circuit for developing the modulation signal whosemean voltage is proportional in polarity and magnitude to the sense andamount of frequency departure of the center frequency of said vwave fromthe assigned center frequency of said diS- criminator, a filter networkcoupled to said output circuit for deriving said mean voltage from saidsignal, circuit means coupled to said discriminat-or for deriving anautomatic gain control voltage representative of the presence or absenceof a carrier wave received by said receiver, a modulation signalamplifier coupled to said output circuit for amplifying said modulationsignal, a first control amplifier responsive to said mean voltage, asecond control amplifier having a lower amplification factor than thatof said signal amplifier and a high mutual conductance and responsive tosaid control voltage and arranged to be conducting in the absence of acarrier wave, said signal amplier and said second control amplifierhaving each a cathode, said cathodes being connected together, animpedance element in the common cathode circuit of said signal amplierand said second control amplifier, said signal amplier being arranged tobe non-conducting in the absence of a carrier wave, whereby the presenceof a carrier Wave will cause said second control amplifier to be cut offand will render said signal amplifier conducting, an electronicindicator tube having a control element and a target, said controlelement being coupled to said first control amplifier for visuallyindicating the magnitude and polarity of said mean voltage, said tubebeing arranged to be inoperative in the absence of a carrier Wave, and acircuit connection between said second control amplifier and said targetto render said tube operative when said second control amplifier is cutofi.

BENJAMIN S. VILKOMERSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,078,055 Carlson et al. Apr. 20,1937 2,096,874 Beers Oct. 26, 1937 2,172,477 Lett Sept. 12,. 19392,261,643 Brown Nov. 4, 1941 2,263,633 Koch Nov. 25, 1941 2,273,098Foster Feb. 17, 1942 2,334,473 Carlson Nov. 16, 1943 2,447,309 BlaisdellAug. 17, 1948 2,451,584 Stone Oct. 19, 1948 2,501,120 Carlson Mar. 21,1950

